linux-kernel - Re: [PATCH v2 3/3] mm/vma: remove mmap() retry merge

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Message-ID: <CAJuCfpHCZS-goD88VhKU1ovuZbOnQk6aWBHnEPu2HsBxfT+xzQ@mail.gmail.com>
Date: Tue, 13 May 2025 08:38:28 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, David Hildenbrand <david@...hat.com>, 
	Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>, 
	Michal Hocko <mhocko@...e.com>, Jann Horn <jannh@...gle.com>, Pedro Falcato <pfalcato@...e.de>, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, Alexander Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v2 3/3] mm/vma: remove mmap() retry merge

On Tue, May 13, 2025 at 6:25 AM Liam R. Howlett <Liam.Howlett@...cle.com> wrote:
>
> * Lorenzo Stoakes <lorenzo.stoakes@...cle.com> [250509 08:14]:
> > We have now introduced a mechanism that obviates the need for a reattempted
> > merge via the mmap_prepare() file hook, so eliminate this functionality
> > altogether.
> >
> > The retry merge logic has been the cause of a great deal of complexity in
> > the past and required a great deal of careful manoeuvring of code to ensure
> > its continued and correct functionality.
> >
> > It has also recently been involved in an issue surrounding maple tree
> > state, which again points to its problematic nature.
> >
> > We make it much easier to reason about mmap() logic by eliminating this and
> > simply writing a VMA once. This also opens the doors to future optimisation
> > and improvement in the mmap() logic.
> >
> > For any device or file system which encounters unwanted VMA fragmentation
> > as a result of this change (that is, having not implemented .mmap_prepare
> > hooks), the issue is easily resolvable by doing so.
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > Reviewed-by: David Hildenbrand <david@...hat.com>
>
> I have a few tests for the vma test suite that would test this path.
> I'll just let them go.
>
> Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>

Reviewed-by: Suren Baghdasaryan <surenb@...gle.com>

>
> > ---
> >  mm/vma.c | 14 --------------
> >  1 file changed, 14 deletions(-)
> >
> > diff --git a/mm/vma.c b/mm/vma.c
> > index 3f32e04bb6cc..3ff6cfbe3338 100644
> > --- a/mm/vma.c
> > +++ b/mm/vma.c
> > @@ -24,7 +24,6 @@ struct mmap_state {
> >       void *vm_private_data;
> >
> >       unsigned long charged;
> > -     bool retry_merge;
> >
> >       struct vm_area_struct *prev;
> >       struct vm_area_struct *next;
> > @@ -2417,8 +2416,6 @@ static int __mmap_new_file_vma(struct mmap_state *map,
> >                       !(map->flags & VM_MAYWRITE) &&
> >                       (vma->vm_flags & VM_MAYWRITE));
> >
> > -     /* If the flags change (and are mergeable), let's retry later. */
> > -     map->retry_merge = vma->vm_flags != map->flags && !(vma->vm_flags & VM_SPECIAL);

Could we have a WARN_ON() here please? I know you took care of in-tree
drivers but if an out-of-tree driver does this there will be no
indication that it has to change its ways. I know we don't care about
out-of-tree ones but they still exist, so maybe we can be nice here
and issue a warning?

> >       map->flags = vma->vm_flags;
> >
> >       return 0;
> > @@ -2622,17 +2619,6 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr,
> >       if (have_mmap_prepare)
> >               set_vma_user_defined_fields(vma, &map);
> >
> > -     /* If flags changed, we might be able to merge, so try again. */
> > -     if (map.retry_merge) {
> > -             struct vm_area_struct *merged;
> > -             VMG_MMAP_STATE(vmg, &map, vma);
> > -
> > -             vma_iter_config(map.vmi, map.addr, map.end);
> > -             merged = vma_merge_existing_range(&vmg);
> > -             if (merged)
> > -                     vma = merged;
> > -     }
> > -
> >       __mmap_complete(&map, vma);
> >
> >       return addr;
> > --
> > 2.49.0
> >